RFR: 8346157: [Ubsan]: runtime error: pointer index expression with base 0x000000001000 overflowed to 0xfffffffffffffff0 [v5]

[Thomas Stuefe]

On Mon, 6 Jan 2025 09:07:36 GMT, Amit Kumar <amitkumar at openjdk.org> wrote:

>> Fixes ubsan warning in mallocTracker.cpp
>
> Amit Kumar has updated the pull request incrementally with one additional commit since the last revision:
> 
>   new year

ok

src/hotspot/share/nmt/mallocTracker.cpp line 247:

> 245:     const size_t smallest_possible_alignment = sizeof(void*);
> 246:     uintptr_t here = (uintptr_t)align_down(addr, smallest_possible_alignment);
> 247:     uintptr_t end = MAX2(smallest_possible_alignment, here - (0x1000 + sizeof(MallocHeader))); // stop searching after 4k

This works since we know that 0x1000 + sizeof(MallocHeader) < 16MB, which is where we bail out above. Otherwise, for here values < 1K+16, we would underflow and end up with a large `end` value.

-------------

Marked as reviewed by stuefe (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22885#pullrequestreview-2549892154
PR Review Comment: https://git.openjdk.org/jdk/pull/22885#discussion_r1914923531